Field of the Invention
The invention relates to a method of driving at least one capacitive actuator by means of a charge voltage.
A method of driving capacitive actuators is disclosed in U.S. Pat. No. 4,688,536, entitled "drive circuit for an electrostrictive actuator in a fuel injection valve. As suggested by the title, the patent relates in particular to a method of driving piezoelectrically operated fuel injection valves of an internal combustion engine. The actuators of the prior patent are charged with constant voltage.
A piezoelectric actuator comprises many piezoceramic layers and forms a so-called "stack", which when a voltage is applied changes its dimensions, and especially its length. In the reverse, the stack generates an electrical voltage in response to mechanical pressure or tensions.
The electrical properties of that kind of piezostack vary with the temperature to which it is exposed. As the temperature rises, its capacitance increases, but its stroke also lengthens. At the temperatures to be taken into account for automotive applications, ranging from about -40 C. to +150 C., changes in capacitance of up to a factor of 2 are observed.
If for instance a piezoelectric actuator is charged at all its operating points with a constant voltage, which at low temperatures brings about the required stroke, then at high temperatures the resultant stroke is markedly longer than necessary. In the context of fuel injection valves with a constant fuel pressure, of course, this means an excessive fuel injection quantity. Since at high temperatures the capacitance of the piezostack is also greater, much more charge and energy are needed than may be otherwise necessary.
Hence the method known from the afore-mentioned U.S. Pat. No. 4,688,536 does not work precisely enough unless the ambient conditions do not change at all (no tolerances in the components used, no changes in the electrical properties, constant temperature).
From U.S. Pat. No. 5,387,834 there is known a drive circuit for a capacitive actuator that is triggered with a constant charge voltage and with a charging time that is determined as a function of the actuator temperature. The temperature is measured with a sensor.